Automatically assist with volunteer invites

[strugee]

Right now, inviting a volunteer is a manual multi-step procedure, because you want to make sure that they're in the staff space, and the all-staff room, and in the general space, and in the general discussion room. Mostly because spaces are confusing in Element if you're not used to them.

Success criteria:

1. There is a command !staffinvite <user pill> or something like that (I'm not attached to this name, feel free to bikeshed) that invites the mentioned user to all the places they should be. The invoking user is required to be a staff member already (which probably just means that they're in the all-staff room already).
2. This command is invoked automatically, in the background, if a user is manually invited to the all-staff room or the staff space.

Bonus points if you do the same for committee rooms and look at the all-staff room's user list to avoid duplicate/unnecessary invites, but since I don't think anyone actually does this in practice, I wouldn't bother.